Field effect transistor and method

ABSTRACT

A device includes a substrate, and a first semiconductor channel over the substrate. The first semiconductor channel includes a first nanosheet of a first semiconductor material, a second nanosheet of a second semiconductor material in physical contact with a topside surface of the first nanosheet, and a third nanosheet of the second semiconductor material in physical contact with an underside surface of the first nanosheet. The first gate structure is over and laterally surrounding the first semiconductor channel, and in physical contact with the second nanosheet and the third nanosheet.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling down has also increased the complexity ofprocessing and manufacturing ICs.

Metal oxide semiconductor (MOS) field effect transistors (FETs), or“MOSFETs,” can be considered voltage-controlled current sources. In aMOSFET, a gate terminal overlies or surrounds a semiconductor channelwhich conducts charged carriers between a source region and a drainregion abutting the semiconductor channel. Voltage at the gate terminal(or “gate voltage”) causes the semiconductor channel to becomeconductive when the gate voltage is above a certain threshold (or“threshold voltage”). As such, the threshold voltage of a MOSFET is theminimum gate-to-source voltage (or source-to-gate voltage) that isneeded to create a conducting path between the source and drain regions.

The threshold voltage is closely related to many MOSFET performancecharacteristics. For example, different functional transistors maybenefit from a variety of threshold voltages. An input-output (IO)transistor may benefit from a high threshold voltage, which allows fordriving a large number of external circuits or circuits that arerelatively distant from the IO transistors. A core logic transistor maybenefit from a low threshold voltage, which allows for faster switching,lower power consumption and/or lower heat generation. As such, greaterIC complexity and functional density have led to a greater variety ofthreshold voltages in a single IC chip.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a diagrammatic cross-sectional side view of a portion of an ICdevice fabricated according to embodiments of the present disclosure.

FIGS. 2A-10F are views of various embodiments of an IC device of atvarious stages of fabrication according to various aspects of thepresent disclosure.

FIG. 11 is a flowchart illustrating a method of fabricating asemiconductor device according to various aspects of the presentdisclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Terms such as “about,” “roughly,” “substantially,” and the like may beused herein for ease of description. A person having ordinary skill inthe art will be able to understand and derive meanings for such terms.For example, about may indicate variation in a dimension of 20%, 10%, 5%or the like, but other values may be used when appropriate. A largefeature, such as the longest dimension of a semiconductor fin may havevariation less than 5%, whereas a very small feature, such as thicknessof an interfacial layer may have variation of as much as 50%, and bothtypes of variation may be represented by the term “about.”“Substantially” is generally more stringent than “about,” such thatvariation of 10%, 5% or less may be appropriate, without limit thereto.A feature that is “substantially planar” may have variation from astraight line that is within 10% or less. A material with a“substantially constant concentration” may have variation ofconcentration along one or more dimensions that is within 5% or less.Again, a person having ordinary skill in the art will be able tounderstand and derive appropriate meanings for such terms based onknowledge of the industry, current fabrication techniques, and the like.

The present disclosure is generally related to semiconductor devices,and more particularly to field-effect transistors (FETs), such as planarFETs, three-dimensional fin-line FETs (FinFETs), or gate-all-around(GAA) FETs. Different threshold voltages (“Vt”) of the semiconductordevices are desirable to optimize performance of circuit elements havingwidely different functional requirements. Performance enhancement andpower reduction are highly desirable for advanced technology nodes, suchthat GAA nanosheet (NS) devices have attracted more attention for bettergate control and higher performance. However, silicon-channel-basednanosheet devices, in which the silicon channel is usually formed (or“released”) by removal of adjacent SiGe layers, also called “sheetformation” (SHF), have generally suffered from lower mobility and higherthreshold voltage due to complexity in the production process. Oneexample is germanium residue which may be present on the siliconchannels post sheet formation, which leads to lower mobility andperformance.

The gate all around (GAA) transistor structures may be patterned by anysuitable method. For example, the structures may be patterned using oneor more photolithography processes, including double-patterning ormulti-patterning processes. Generally, double-patterning ormulti-patterning processes combine photolithography and self-alignedprocesses, allowing patterns to be created that have, for example,pitches smaller than what is otherwise obtainable using a single, directphotolithography process. For example, in one embodiment, a sacrificiallayer is formed over a substrate and patterned using a photolithographyprocess. Spacers are formed alongside the patterned sacrificial layerusing a self-aligned process. The sacrificial layer is then removed, andthe remaining spacers may then be used to pattern the GAA structure.

In a typical GAA NS device, a semiconductor lattice includingalternating layers of Si and SiGe is formed, after which the SiGe layersare removed to form the Si nanosheets, or simply “nanosheets.” Inprocesses with thermal budget, germanium from the SiGe nanosheets, whichgenerally employ a high germanium concentration to ensure breakthroughduring release, intermixes with the Si nanosheets, which leads tosubstantial germanium residue after sheet formation, even in thepresence of high etching selectivity. Germanium in or on the nanosheettends to increase threshold voltage in absolute terms, e.g., shiftingthreshold voltage in the positive direction by some significant amount.For an N-type FET (NFET), the positive shift corresponds to an increasein magnitude of the threshold voltage. For a P-type FET (PFET), whichtypically has a threshold voltage that is a negative value, the positiveshift corresponds to a decrease in magnitude of the threshold voltage.

An improved semiconductor lattice is disclosed herein which employs amulti-layer SiGe nanosheet structure between the Si nanosheets toprovide selective inclusion/exclusion of SiGe material from the Sinanosheet. A middle layer of the SiGe nanosheet includes a high Geconcentration (atomic % as much as 40% in some embodiments), whichensures sheet formation breakthrough during removal. Side layers, suchas a top layer and a bottom layer overlying and underlying the middlelayer, respectively, include a lower Ge concentration (atomic % aslittle as 10% in some embodiments), which reduces intermixing betweenthe silicon of the Si nanosheets and the germanium of the SiGe nanosheetstructures during thermal processes, such as annealing. Following sheetformation, most or all remaining SiGe from the side layers may beremoved by a trimming process, such as a SiGe oxidation process followedby oxide removal. As such, the Si nanosheets are formed substantiallyfree of germanium, which allows for higher mobility and performance. Insome embodiments, a portion or the entirety of each side layer isintentionally left on the Si nanosheets, which can give the addedbenefit of lowering threshold voltage magnitude for certain P-type FETsor increasing the threshold voltage magnitude of certain N-type FETs,thereby providing an additional lever for threshold voltage tuning.

FIG. 1 illustrates a diagrammatic cross-sectional side view of a portionof an IC device 10 fabricated according to embodiments of the presentdisclosure, where the IC device 10 includes gate-all-around (GAA)devices 20N, 20P. The GAA devices 20N, 20P may include at least an NFETor a PFET in some embodiments. For example, the GAA device 20N is anNFET, and the GAA device 20P is a PFET, in accordance with someembodiments. Integrated circuit devices such as the IC device 10frequently include transistors having different threshold voltages basedon their function in the IC device. High-performance, core logictransistors typically have the lowest threshold voltages (magnitude) toachieve higher switching speeds at lower operating power. Input/output(IO) transistors typically have the highest threshold voltages due tothe high current handling required of the IO transistors. Otherthreshold voltages between that of the IO transistors and that of thecore logic transistors may also be employed for certain other functionaltransistors, such as static random access memory (SRAM) transistors.Some functional circuit blocks within the IC device 10 may include twoor more NFETs and/or PFETs of two or more different threshold voltages.

The cross-sectional view of the IC device 10 in FIG. 1 is taken along anX-Z plane, where the X-direction is the horizontal direction, and theZ-direction is the vertical direction. The GAA devices 20N, 20P eachinclude channels 22A-22C (alternately referred to as “nanostructures”)over a fin structure 32. The channels 22A-22C are laterally abutted bysource/drain features 82, and covered and surrounded by gate structures200A, 200B. The gate structures 200A, 200B control flow of electricalcurrent through the channels 22A-22C based on voltages applied at thegate structures 200A, 200B and at the source/drain features 82. Thethreshold voltage is a minimum voltage (e.g., gate-source voltage orsource-gate voltage) needed to establish a conducting path in thechannels 22A-22C. Threshold voltage enhancement and/or tuning duringfabrication of the various transistors, e.g., IO transistors, core logictransistors, and SRAM transistors, preferably with low modification ofthe fabrication process, is accomplished by at least one of thetechniques applied during fabrication of the gate structures 200A, 200B,described in greater detail below.

In some embodiments, the fin structure 32 includes silicon. In someembodiments, the GAA device 20N is an NFET, and the source/drainfeatures 82 thereof include silicon phosphorous (SiP). In someembodiments, the GAA device 20P is a PFET, and the source/drain features82 thereof include silicon germanium (SiGe).

The channels 22A-22C each include a semiconductive material, for examplesilicon or a silicon compound, such as silicon germanium, or the like.The channels 22A-22C have nanostructures (e.g., having sizes that are ina range of a few nanometers) and may also each have an elongated shapeand extend in the X-direction. In some embodiments, the channels 22A-22Ceach have a nanowire (NW) shape, a nanosheet (NS) shape, a nanotube (NT)shape, or other suitable nanoscale shape. The cross-sectional profile ofthe channels 22A-22C may be rectangular, round, square, circular,elliptical, hexagonal, or combinations thereof. In most embodiments, thechannels 22A-22C include silicon nanosheets.

In some embodiments, the lengths (e.g., measured in the X-direction) ofthe channels 22A-22C may be different from each other, for example dueto tapering during a fin etching process. In some embodiments, length ofthe channel 22A may be less than a length of the channel 22B, which maybe less than a length of the channel 22C. The channels 22A-22C each maynot have uniform thickness, for example due to a channel trimmingprocess used to expand spacing (e.g., measured in the Z-direction)between the channels 22A-22C to increase gate structure fabricationprocess window. For example, a middle portion of each of the channels22A-22C may be thinner than the two ends of each of the channels22A-22C. Such shape may be collectively referred to as a “dog-bone”shape.

In some embodiments, each of the channels 22A-22C is simply thenanosheet 225A-225C, respectively, such as is illustrated in the GAAdevice 20N. In some embodiments, each of the channels 22A-22C mayfurther include a low-concentration silicon germanium (SiGe) layer 242on a topside and/or a low-concentration SiGe layer 243 on an undersideof each of the silicon nanosheets 225A-225C. In some embodiments,topsides of the fins 32 also include low-concentration SiGe layers 242.For example, as shown in FIG. 1 , the GAA device 20P, which may be aPFET core logic transistor, includes the low-concentration SiGe layers242, 243 on topsides and/or undersides of the silicon nanosheets225A-225C, and the topside of the fin 32, respectively. Thelow-concentration SiGe layers 242, 243 reduce the threshold voltage(magnitude) in PFETs, e.g., by shifting the threshold voltage in thepositive voltage direction, which may be desirable for core logictransistors, which generally employ very low threshold voltages. In thepreceding description, reference to topside generally indicates a sideof a structure most distant from the major surface of the substrate onwhich the structure is formed, and reference to underside generallyindicates a side of the structure nearest the major surface of thesubstrate. For example, in the GAA device 20P, the low-concentrationSiGe layer 242B is on the topside of the nanosheet 225B of the GAAdevice 20P, and the low-concentration SiGe layer 243B is on theunderside of the nanosheet 225B.

In some embodiments, the low-concentration SiGe layers 242, 243 includegermanium at atomic percentage in a range of about 10% to about 25%. Assuch, each low-concentration SiGe layer 242, 243 may be Si_(1-x)Ge_(x)that includes about 10% to about 25% (0.1<x<0.25) Ge in molar ratio.Concentration of germanium may be substantially constant within each ofthe low-concentration SiGe layers 242, 243. In some embodiments, theconcentration has variation less than about 10% along the height(Z-direction) of the low-concentration SiGe layer 242, 243. This may bedue to formation of the low-concentration SiGe layers 242, 243 throughan epitaxial process rather than through intermixing between the SiGenanosheets 23 and the silicon nanosheets 21 (FIGS. 2A-2C) or the SiGenanosheets 24 and the silicond nanosheets 22 (FIGS. 3A-3C) that wouldoccur in a thermal process, such as an anneal, in conventionalapproaches.

In some embodiments, the spacing between nanosheets 225A-225C of thechannels 22A-22C (e.g., between the nanosheet 225B and the nanosheet225A or the nanosheet 225C) is in a range between about 7 nanometers(nm) and about 13 nm. In some embodiments, a thickness (e.g., measuredin the Z-direction) of each of the nanosheets 225A-225C is in a rangebetween about 4 nm and about 8 nm. In some embodiments, a width (e.g.,measured in the Y-direction, not shown in FIG. 1 , orthogonal to the X-Zplane) of each of the nanosheets 225A-225C is at least about 8 nm. Insome embodiments, a thickness (e.g., measured in the Z-direction) of thelow-concentration SiGe layers 242, 243 is in a range of about 0.5 nm toabout 2 nm. In some embodiments, the thickness of the nanosheets 225 isgreater than the thickness of each of the low-concentration SiGe layers242, 243 by a multiple in a range of about 2 to about 16. The multiplebeing less than about 2 means that the combined thickness of thelow-concentration SiGe layers 242, 243 is greater than the thickness ofthe nanosheet 225 in the channel 22, which is undesirable due toperformance of the channel 22 being dominated by the material of thelow-concentration SiGe layers 242, 243 rather than the nanosheet 225.The multiple being greater than about 16 may cause insufficientisolation between the nanosheet 225 and a high-concentration SiGe layer231, 241 (see FIG. 2C and FIG. 3C), such that heavy intermixing of Geand Si occurs near the top and bottom surfaces of the nanosheet 225.

In some embodiments, the spacing between the nanosheets is greater thanthe thickness of each of the low-concentration SiGe layers 242, 243 by amultiple in a range of about 3.5 to about 26. The multiple being lessthan about 3.5 may impinge upon gate fill window. The multiple beinggreater than 26 may cause insufficient isolation between the nanosheet225 and a high-concentration SiGe layer 231, 241 (see FIG. 2C and FIG.3C), such that heavy intermixing of Ge and Si occurs near the top andbottom surfaces of the nanosheet 225.

In some embodiments, thickness of the channels 22A-22C of the GAA device20P, is in a range between about 5 nm to about 12 nm. In someembodiments, thickness of the channels 22A-22C of the GAA device 20P isnot uniform in the X-direction due to the low-concentration SiGe layers242, 243, such that the thickness of the channels 22A-22C at theperipheries (e.g., vertically between the inner spacers) is in a rangebetween about 4 nm to about 8 nm, and the thickness of the channels22A-22C between the peripheries is in a range between about 5 nm toabout 12 nm.

The channels 22A are uppermost channels of the GAA devices 20N, 20P,most distant from the substrate 110. In some embodiments, the nanosheet225A of the GAA device 20P is not covered by a SiGe layer structureprior to nanosheet formation. In such embodiments, the topside of thenanosheet 225A of the GAA device 20P may be substantially free of SiGe.As such, the thickness of the channel 22A of the GAA device 20P may bein a range of about 4.5 nm to about 10 nm.

The gate structures 200A, 200B, are disposed over and between thechannels 22A-22C, respectively. A first interfacial layer (IL) 210,which may be an oxide of the material of the channels 22A-22C, is formedon exposed areas of the channels 22A-22C and the top surface of the fin32. The first IL layer 210 promotes adhesion of the gate dielectriclayer 220 to the channels 22A-22C. In some embodiments, the first ILlayer 210 has thickness of about 5 Angstroms (A) to about 50 Angstroms(A). In some embodiments, the first IL layer 210 has thickness of about10 A. The first IL layer 210 having thickness that is too thin mayexhibit voids or insufficient adhesion properties. The first IL layer210 being too thick consumes gate fill window.

The gate dielectric layer 220, which may be a multi-layer structure,includes a high-k gate dielectric material in some embodiments, whichmay refer to dielectric materials having a high dielectric constant thatis greater than a dielectric constant of silicon oxide (k≈3.9).Exemplary high-k dielectric materials include HfO₂, HfSiO, HfSiON,HfTa0, HfSiO, HfZrO, ZrO₂, Ta₂O₅, or combinations thereof. In someembodiments, the gate dielectric layer 220 has thickness of about 5 A toabout 50 A, which may be similar to, or somewhat thicker than, the firstIL layer 210. In some embodiments, the gate dielectric layer 220 hasthickness of about 15 A, or about half again the thickness of the firstIL layer 210.

The gate structures 200A, 200B further include one or more work functionmetal layers, represented collectively as the work function metal layer900. In the GAA device 20N, which is an NFET in most embodiments, thefirst work function metal layer 900 may include at least an N-type workfunction metal layer, an in-situ capping layer, and an oxygen blockinglayer. In some embodiments, the first work function metal layer 900includes more or fewer layers than those described. In the GAA device20P, which is a PFET in most embodiments, the first work function metallayer 900 may be substantially the same as in the GAA device 20N.

In some embodiments, the gate structures 200A, 200B further include asecond IL 240, which promotes better metal gate adhesion on the gatedielectric layer 220, e.g. between the gate dielectric layer 220 and thework function metal layer 900. In many embodiments, the second IL 240further provides improved thermal stability for the gate structures200A-200B, and serves to limit diffusion of metallic impurity from thework function metal layers 900 to the gate dielectric layer 220. Thesecond IL 240 may be or comprise TiSiNO, in some embodiments.

The gate structures 200A, 200B also include metal fill layers 290. Themetal fill layers 290 may include a conductive material such astungsten, cobalt, ruthenium, iridium, molybdenum, copper, aluminum, orcombinations thereof. Between the channels 22A-22C, the metal filllayers 290 are circumferentially surrounded (in the cross-sectionalview) by the one or more work function metal layers 900, which are thencircumferentially surrounded by the gate dielectric layer 220. In theportion of the gate structures 200A, 200B formed over the channel 22Amost distal from the fin 32, the metal fill layers 290 are formed overthe work function metal layer 900. The work function metal layer 900wraps around the metal fill layer 290. The gate dielectric layer 220also wraps around the work function metal layer 900. The gate structures200A, 200B may also include a glue layer that is formed between the workfunction metal layer 900 and the metal fill layer 290 to increaseadhesion. The glue layer is not specifically illustrated in FIG. 1 forsimplicity.

The GAA devices 20N, 20P also include gate spacers 41 and inner spacers74 that are disposed on sidewalls of the gate dielectric layer 220. Theinner spacers 74 are also disposed between the channels 22A-22C. Thegate spacers 41 and the inner spacers 74 may include a dielectricmaterial, for example a low-k material such as SiOCN, SiON, SiN, orSiOC. In some embodiments, due to inclusion of the low-concentrationSiGe layers 242, 243 prior to nanosheet formation, the inner spacers 74have notches facing the gate structure 200A or 200B. Formation ofnotches is described in greater detail in the discussion of FIG. 6Drelated to channel recessing in preparation for inner spacer formation.Generally, due to inclusion of the low-concentration SiGe layers 242,243 in the SiGe nanosheets 23 (see FIGS. 2A-2C) between the siliconnanosheets 225A-225C, etch rate of high-concentration SiGe layers 231(FIG. 2C) is much greater than etch rate of the low-concentration SiGelayers 242, 243, which is in turn much greater than etch rate of thesilicon nanosheets 225A-225C, which results in steps between the abovelayers, as shown in FIG. 6D. When the inner spacers 74 are formed, theinner spacers 74 generally adopt the stepped shape of the openings 64(FIG. 6D) in which the inner spacers 74 are formed.

The GAA devices 20N, 20P further include source/drain contacts 120 thatare formed over the source/drain features 82. The source/drain contacts120 may include a conductive material such as tungsten, cobalt,ruthenium, iridium, molybdenum, copper, aluminum, or combinationsthereof. The source/drain contacts 120 may be surrounded by barrierlayers (not shown), such as SiN or TiN, which help prevent or reducediffusion of materials from and into the source/drain contacts 120. Asilicide layer 118 may also be formed between the source/drain features82 and the source/drain contacts 120, so as to reduce the source/draincontact resistance. The silicide layer 118 may contain a metal silicidematerial, such as cobalt silicide in some embodiments, or TiSi in someother embodiments.

The GAA devices 20N, 20P further include an interlayer dielectric (ILD)130. The ILD 130 provides electrical isolation between the variouscomponents of the GAA devices 20N, 20P discussed above, for examplebetween the gate structures 200A, 200B and the source/drain contacts120.

Additional details pertaining to the fabrication of GAA devices aredisclosed in U.S. Pat. No. 10,164,012, titled “Semiconductor Device andManufacturing Method Thereof” and issued on Dec. 25, 2018, as well as inU.S. Pat. No. 10,361,278, titled “Method of Manufacturing aSemiconductor Device and a Semiconductor Device” and issued on Jul. 23,2019, the disclosures of each which are hereby incorporated by referencein their respective entireties.

FIG. 11 illustrates a flowchart illustrating a method 1000 for formingan IC device or a portion thereof from a workpiece, according to one ormore aspects of the present disclosure. Method 1000 is merely an exampleand is not intended to limit the present disclosure to what isexplicitly illustrated in method 1000. Additional acts can be providedbefore, during and after the method 1000, and some acts described can bereplaced, eliminated, or moved around for additional embodiments of themethods. Not all acts are described herein in detail for reasons ofsimplicity. Method 1000 is described below in conjunction withfragmentary cross-sectional views of a workpiece (shown in FIGS. 2A-2B,3A-3B, 4A-4C, 5A-5C, 6A-6C, 7A-7C, 8A-8C, 9A-9C, and 10A-10C) atdifferent stages of fabrication according to embodiments of method 1000.For avoidance of doubt, throughout the figures, the X direction isperpendicular to the Y direction and the Z direction is perpendicular toboth the X direction and the Y direction. It is noted that, because theworkpiece may be fabricated into a semiconductor device, the workpiecemay be referred to as the semiconductor device as the context requires.

FIGS. 2A through 10F are perspective views and cross-sectional views ofintermediate stages in the manufacturing of nano-FETs, in accordancewith some embodiments. FIGS. 2A, 3A, 4A, 5A, 6A, 7A, 8A, 9A, and 10Aillustrate perspective views. FIGS. 2B, 3B, 4B, 5B, 6B, 7B, 8B, 9B, and10B illustrate reference cross-section B-B′ (gate cut) illustrated inFIGS. 2A, 3A, and 4A. FIGS. 4C, 5C, 6C, 7C, 8C, 9C, and 10C illustratereference cross-section C-C′ (channel/fin cut) illustrated in FIG. 4A.FIGS. 2C and 3C illustrate enlarged cross-sectional views alongcross-sectional line B-B′ illustrated in FIGS. 2A, 3A and 4A. FIGS. 6D,7D, 9D, 9E, 9F, 10D, 10E and 10F illustrate enlarged cross-sectionalviews along cross-sectional line C-C′ illustrated in FIG. 4A.

In FIG. 2A and FIG. 2B, a substrate 110 is provided. The substrate 110may be a semiconductor substrate, such as a bulk semiconductor, or thelike, which may be doped (e.g., with a p-type or an n-type dopant) orundoped. The semiconductor material of the substrate 110 may includesilicon; germanium; a compound semiconductor including silicon carbide,gallium arsenide, gallium phosphide, indium phosphide, indium arsenide,and/or indium antimonide; an alloy semiconductor includingsilicon-germanium, gallium arsenide phosphide, aluminum indium arsenide,aluminum gallium arsenide, gallium indium arsenide, gallium indiumphosphide, and/or gallium indium arsenide phosphide; or combinationsthereof. Other substrates, such as single-layer, multi-layered, orgradient substrates may be used.

Further in FIG. 2A and FIG. 2B, a multi-layer stack 25 or “lattice” isformed over the substrate 110 of alternating layers of firstsemiconductor layers 21A-21C (collectively referred to as firstsemiconductor layers 21) and second semiconductor layer stacks 23A-23C(collectively referred to as second semiconductor layer stacks 23). Insome embodiments, the first semiconductor layers 21 may be formed of afirst semiconductor material suitable for n-type nano-FETs, such assilicon, silicon carbide, or the like, and the second semiconductorlayer stacks 23 may be formed of a second semiconductor materialsuitable for p-type nano-FETs, such as silicon germanium or the like.Each of the layers of the multi-layer stack 25 may be epitaxially grownusing a process such as chemical vapor deposition (CVD), atomic layerdeposition (ALD), vapor phase epitaxy (VPE), molecular beam epitaxy(MBE), or the like.

Three layers of each of the first semiconductor layers 21 and the secondsemiconductor layer stacks 23 are illustrated. In some embodiments, themulti-layer stack 25 may include one or two each or four or more each ofthe first semiconductor layers 21 and the second semiconductor layerstacks 23. Although the multi-layer stack 25 is illustrated as includinga second semiconductor layer 23C as the bottommost layer, in someembodiments, the bottommost layer of the multi-layer stack 25 may be afirst semiconductor layer 21.

The first semiconductor materials and the second semiconductor materialsmay be chosen to have high etch selectivity to each other. The high etchselectivity allows the second semiconductor layer stacks 23 of thesecond semiconductor material to be removed without significantlyremoving the first semiconductor layers 21 of the first semiconductormaterial, thereby allowing the first semiconductor layers 21 to bepatterned to form channel regions of nano-FETs. In conventionalapproaches, thermal processes cause intermixing of the firstsemiconductor materials and the second semiconductor materials atinterfaces between the first semiconductor materials and the secondsemiconductor materials. As such, the intermixing can lead to germaniumresidue in silicon nanosheets, which causes potentially undesirablelower mobility and positive shift in threshold voltage.

Referring to FIG. 2C, each second semiconductor layer stack 23 includesa high-concentration SiGe layer 231 stacked between a topsidelow-concentration SiGe layer 232 and an underside low-concentration SiGelayer 233. For example, the second semiconductor layer stack 23Bincludes the high-concentration SiGe layer 231B stacked between thetopside low-concentration SiGe layer 232B and the undersidelow-concentration SiGe layer 233B. In some embodiments, an additionallow-concentration SiGe layer 233D is formed atop the topmost firstsemiconductor layer 21A. The high-concentration SiGe layers 231 havegermanium atomic percent (concentration) higher than in thelow-concentration SiGe layers 232, 233, such as in a range of about 20%to about 40%. The high-concentration SiGe layers 231 ensure breakthroughduring nanosheet formation, and the low-concentration SiGe layers 232,233 significantly reduce or eliminate intermixing of silicon in thesilicon nanosheets 21A with germanium that would normally occur if thelow-concentration SiGe layers 232, 233 were not present. Below about 20%germanium concentration in the high-concentration SiGe layers 231 maylead to incomplete breakthrough in nanosheet formation, which isundesirable in many aspects (e.g., at least threshold voltage shift,gate fill window, channel separation). The low-concentration SiGe layers232, 233 have germanium atomic percent (concentration) lower than in thehigh-concentration SiGe layers 231, such as in a range of about 10% toabout 25%. Germanium concentration greater than 25% in thelow-concentration SiGe layers 232, 233 may also cause undesirableeffects, such as heavy intermixing of Ge with Si in the firstsemiconductor layers 21. In some embodiments, the germaniumconcentration in the high-concentration SiGe layers 231 is greater thanthe germanium concentration in the low-concentrations SiGe layers by atleast about 5%.

In some embodiments, each second semiconductor layer stack 23 is formedby a single, continuous epitaxial growth process in which constituentgasses are mixed in varying ratios to form first the undersidelow-concentration SiGe layer 233, followed by the high-concentrationSiGe layer 231, followed by the topside low-concentration SiGe layer232. In some embodiments, each of the underside low-concentration SiGelayers 233, each of the high-concentration SiGe layers 231, and each ofthe topside low-concentration SiGe layers 232 is formed in an individualepitaxial growth process. The epitaxial growth can use CVD, MOCVD, MBE,LPE, VPE, UHVCVD, or the like, or a combination thereof. By way ofexample, precursor gases that can be used for the growth of the SiGelayers include silane (SiH4), disilane (Si2H6), germane (GeH4),digermane (Ge2H6), dichlorosilane (SiH2Cl2), any other suitable gas, orcombinations thereof. In some embodiments, each or individual epitaxialgrowth processes are performed in the presence of hydrogen (H2) gas. Insome embodiments, each low-concentration SiGe layer 232, 233 is formedto have germanium atomic percent (concentration) in a range of about 10%to about 25% by mixing a first germanium precursor (e.g., GeH4) and afirst silicon precursor (e.g., Si2H6) in a first ratio (e.g.,GeH4/Si2H6). In some embodiments, each high-concentration SiGe layer 231is formed to have germanium atomic percent (concentration) higher thanin the low-concentration SiGe layers 232, 233, such as in a range ofabout 20% to about 40%. As such, a second ratio of the first germaniumprecursor to the first silicon precursor used for forming thehigh-concentration SiGe layers 231 is higher than the first ratio usedfor forming the low-concentration SiGe layers 232, 233.

Taking the second semiconductor layer stack 23A for purposes ofillustration, following growth of the first semiconductor layer 21B, afirst epitaxial growth process is performed using the first germaniumprecursor and the first silicon precursor in the first ratio for a firstduration to form the underside low-concentration SiGe layer 233A. Asecond epitaxial growth process, which may follow immediately after thefirst epitaxial growth process, is performed using the second ratio fora second duration to form the high-concentration SiGe layer 231A. Athird epitaxial growth process, which may follow immediately after thesecond epitaxial growth process, is performed using a third ratio for athird duration to form the topside low-concentration SiGe layer 232A. Insome embodiments, the third ratio is substantially the same as the firstratio (e.g., within <5% variation). In some embodiments, the thirdduration is substantially the same as the first duration (e.g., within<5% variation). As such, the topside low-concentration SiGe layer 232Ahas substantially the same characteristics (thickness, Ge concentration)as the underside low-concentration SiGe layer 233A. In some embodiments,the second ratio is higher than the first and third ratios, such thatthe germanium concentration in the high-concentration SiGe layers 231 isgreater than the germanium concentration in the low-concentrations SiGelayers by at least about 5%. In some embodiments, the second duration islonger than the first and third durations, such that thehigh-concentration SiGe layer 231A is thicker than the topside andunderside low-concentration SiGe layers 232A, 233A by a multiple in arange of about 2.5 to about 10. Above about 10, the low-concentrationSiGe layers 232, 233 may be insufficiently thick to prevent intermixingfrom the high-concentration SiGe layers 231 to the first semiconductorlayers 21. Below about 2.5, the high-concentration SiGe layers 231 maybe insufficiently thick to provide adequate separation between thenanosheets 225A-225C and/or provide adequate breakthrough duringnanosheet formation.

FIG. 2C illustrates a first region 100N and a second region 100P of thesubstrate 110. In some embodiments, the first region 100N is a region ofthe substrate 110 in and/or on which NFET devices are formed. In someembodiments, the second region 100P is a region of the substrate 110 inand/or on which PFET devices are formed. While shown as neighboring(adjacent) regions in FIG. 2C, the first region 100N and the secondregion 100P may also be separated by a distance or number of fins.

In FIG. 3A and FIG. 3B, fins 32 are formed in the substrate 110 andnanostructures 225, 24 are formed in the multi-layer stack 25corresponding to act 1100 of FIG. 11 . In some embodiments, thenanostructures 225, 24 and the fins 32 may be formed by etching trenchesin the multi-layer stack 25 and the substrate 110. The etching may beany acceptable etch process, such as a reactive ion etch (RIE), neutralbeam etch (NBE), the like, or a combination thereof. The etching may beanisotropic. First nanostructures 225A-225C (also referred to as“nanosheets”) are formed from the first semiconductor layers 21, andsecond nanostructures 24A-24C are formed from the second semiconductorlayers 23. Distance CD1 between adjacent fins 32 and nanostructures 225,24 may be from about 18 nm to about 100 nm.

The fins 32 and the nanostructures 225, 24 may be patterned by anysuitable method. For example, one or more photolithography processes,including double-patterning or multi-patterning processes, may be usedto form the fins 32 and the nanostructures 225, 24. Generally,double-patterning or multi-patterning processes combine photolithographyand self-aligned processes, allowing for pitches smaller than what isotherwise obtainable using a single, direct photolithography process. Asan example of one multi-patterning process, a sacrificial layer mayformed over a substrate and patterned using a photolithography process.Spacers are formed alongside the patterned sacrificial layer using aself-aligned process. The sacrificial layer is then removed, and theremaining spacers may then be used to pattern the fins 32.

FIGS. 3A and 3B illustrate the fins 32 having tapered sidewalls, suchthat a width of each of the fins 32 and/or the nanostructures 225, 24continuously increases in a direction towards the substrate 110. In suchembodiments, each of the nanostructures 225, 24 may have a differentwidth and be trapezoidal in shape. In other embodiments, the sidewallsare substantially vertical (non-tapered), such that width of the fins 32and the nanostructures 225, 24 is substantially similar, and each of thenanostructures 225, 24 is rectangular in shape.

In FIGS. 3A and 3B, isolation regions 36, which may be shallow trenchisolation (STI) regions, are formed adjacent the fins 32. The isolationregions 36 may be formed by depositing an insulation material over thesubstrate 110, the fins 32, and the nanostructures 225, 24, and betweenadjacent fins 32 and nanostructures 225, 24. The insulation material maybe an oxide, such as silicon oxide, a nitride, the like, or acombination thereof, and may be formed by high-density plasma CVD(HDP-CVD), flowable CVD (FCVD), the like, or a combination thereof. Insome embodiments, a liner (not separately illustrated) may first beformed along surfaces of the substrate 110, the fins 32, and thenanostructures 225, 24. Thereafter, a fill material, such as thosediscussed above may be formed over the liner. In some embodiments, afterdeposition of the insulation material, a thermal process, such as ananneal operation, may be performed, for example, to improve the qualityof the insulation material. In some embodiments, the insulation material(and subsequently formed isolation regions 36) may include a multi-layerstructure, for example, having one or more liner layers.

The insulation material undergoes a removal process, such as a chemicalmechanical polish (CMP), an etch-back process, combinations thereof, orthe like, to remove excess insulation material over the nanostructures225, 24. Top surfaces of the nanostructures 225 may be exposed and levelwith the insulation material after the removal process is complete.

The insulation material is then recessed to form the isolation regions36. After recessing, the nanostructures 225, 24 and upper portions ofthe fins 32 may protrude from between neighboring isolation regions 36.The isolation regions 36 may have top surfaces that are flat asillustrated, convex, concave, or a combination thereof. In someembodiments, the isolation regions 36 recessed by an acceptable etchingprocess, such as an oxide removal using, for example, dilutehydrofluoric acid (dHF), which is selective to the insulation materialand leaves the fins 32 and the nanostructures 225, 24 substantiallyunaltered.

FIGS. 2A through 3B illustrate one embodiment (e.g., etch last) offorming the fins 32 and the nanostructures 225, 24. In some embodiments,the fins 32 and/or the nanostructures 225, 24 are epitaxially grown intrenches in a dielectric layer. The epitaxial structures may comprisethe alternating semiconductor materials discussed above, such as thefirst semiconductor materials and the second semiconductor materials.

Further in FIG. 3A and FIG. 3B, appropriate wells (not separatelyillustrated) may be formed in the fins 32, the nanostructures 225, 24,and/or the isolation regions 36. Using masks, an n-type impurity implantmay be performed in p-type regions of the substrate 110, and a p-typeimpurity implant may be performed in n-type regions of the substrate110. Example n-type impurities may include phosphorus, arsenic,antimony, or the like. Example p-type impurities may include boron,boron fluoride, indium, or the like. An anneal may be performed afterthe implants to repair implant damage and to activate the p-type and/orn-type impurities. In some embodiments, in situ doping during epitaxialgrowth of the fins 32 and the semiconductor layers 21, 23 may obviateseparate implantations, although in situ and implantation doping may beused together.

Referring to FIG. 3C, following fin formation and isolation regionformation, as described above in relation to FIGS. 3A and 3B, the firstsemiconductor layers 21 are patterned to form the nanosheets 225. Thetopside low-concentration SiGe layers 232A-232C are patterned to formthe underside low-concentration SiGe layers 243A-243C corresponding tothe nanosheets 225A-225C, respectively. The underside low-concentrationSiGe layers 233A, 233B are patterned to form the topsidelow-concentration SiGe layers 242B, 242C corresponding to the nanosheets225B, 225C. The low-concentration SiGe layer 233D is patterned to formthe topside low-concentration SiGe layers 242A corresponding to thenanosheets 225A. The low-concentration SiGe layer 233C is patterned toform the topside low-concentration SiGe layers 242D corresponding to thefins 32.

Due to the thermal processes, for example, the anneal performed inisolation region formation and/or following impurity implant, each ofthe nanosheets 225A-225C may include topside and/or undersideintermixing regions 221, 222. The fin 32 may also include a topsideintermixing region 321. The topside and underside intermixing regions221, 321, 222 may be collectively referred to as “intermixing regions.”The intermixing regions 221, 321, 222 may be formed quite thin, e.g.,3-5 Angstroms, compared to conventional processes, as a result of therelatively thin low-concentration SiGe layers 232, 233 having Geconcentration between about 10% and about 25%. Furthermore,concentration of Ge in the intermixing regions 221, 321, 222 may be lessthan about 5%. In some embodiments, the intermixing regions 221, 321,222 are substantially free of Ge, e.g., having Ge concentration lessthan about 0.1%. Ge concentration in the intermixing regions 221, 321,222 greater than about 5% may cause an undesirable shift in thresholdvoltage in the GAA device 20N.

In FIGS. 4A-4C, dummy gate structures 40 are formed over the fins 32and/or the nanostructures 225, 24, corresponding to act 1200 of FIG. 11. A dummy gate layer 45 is formed over the fins 32 and/or thenanostructures 225, 24. The dummy gate layer 45 may be made of materialsthat have a high etching selectivity versus the isolation regions 36.The dummy gate layer 45 may be a conductive, semiconductive, ornon-conductive material and may be selected from a group includingamorphous silicon, polycrystalline-silicon (polysilicon),poly-crystalline silicon-germanium (poly-SiGe), metallic nitrides,metallic silicides, metallic oxides, and metals. The dummy gate layer 45may be deposited by physical vapor deposition (PVD), CVD, sputterdeposition, or other techniques for depositing the selected material. Amask layer 47 is formed over the dummy gate layer 45, and may include,for example, silicon nitride, silicon oxynitride, or the like. In someembodiments, a gate dielectric layer (not illustrated for simplicity) isformed before the dummy gate layer 45 between the dummy gate layer 45and the fins 32 and/or the nanostructures 225, 24.

A spacer layer 41 is formed over sidewalls of the mask layer 47 and thedummy gate layer 45. The spacer layer 41 is made of an insulatingmaterial, such as silicon nitride, silicon oxide, silicon carbo-nitride,silicon oxynitride, silicon oxy carbo-nitride, or the like, and may havea single-layer structure or a multi-layer structure including aplurality of dielectric layers, in accordance with some embodiments. Thespacer layer 41 may be formed by depositing a spacer material layer (notshown) over the mask layer 47 and the dummy gate layer 45. Portions ofthe spacer material layer between dummy gate structures 40 are removedusing an anisotropic etching process, in accordance with someembodiments.

In FIGS. 5A-5C, an etching process is performed to etch the portions ofprotruding fins 32 and/or nanostructures 225, 24 that are not covered bydummy gate structures 40, resulting in the structure shown. Therecessing may be anisotropic, such that the portions of the fins 32directly underlying the dummy gate structures 40 and the spacer layer 41are protected, and are not etched. The top surfaces of the recessed fins32 may be substantially coplanar with the top surfaces of the isolationregions 36 as shown, in accordance with some embodiments. The topsurfaces of the recessed fins 32 may be lower than the top surfaces ofthe isolation regions 36, in accordance with some other embodiments.

FIGS. 6A-6D and 7A-7D illustrate formation of inner spacers 74corresponding to act 1300 of FIG. 11 . Generally, a selective etchingprocess is performed to recess end portions of the nanostructures 24exposed by openings in the spacer layer 41 without substantiallyattacking the nanostructures 225, as shown in FIG. 6C. After theselective etching process, recesses 64 are formed in the nanostructures24 at locations where the removed end portions used to be. The resultingstructure is shown in FIGS. 6A-6C.

Referring to FIG. 6D, an enlarged view showing detailed structure of therecesses 64 is shown. Each of the recesses 64 is located either betweenvertically neighboring nanosheets 225, e.g., between the nanosheet 225Aand the nanosheet 225B, or between the fin 32 and the nanosheet 225closest the fin 32, e.g., the nanosheet 225C as shown in FIG. 6D. Thefins 32, the nanosheets 225, the low-concentration SiGe layers 242, 243,and the high-concentration SiGe layers 241 each have different etchingselectivity due to their differing Ge concentrations. Thehigh-concentration SiGe layers 241 are etched the fastest, followed bythe low-concentration SiGe layers 242, followed by the fins 32 and thenanosheets 225, which may be substantially unetched due to beingsubstantially free of Ge, or may be etched only slightly at theintermixing regions 221, 222, 321 when present. As such, thehigh-concentration SiGe layers 241 are etched to a first depth W1,corresponding to lateral distance between an outer sidewall of thenanosheet 225 and an outer sidewall of the high-concentration SiGe layer241. The low-concentration SiGe layers 242, 243 are etched to seconddepths W2, W3, respectively, corresponding to lateral distance betweenthe outer sidewall of the nanosheet 225 and an outer sidewall of thelow-concentration SiGe layer 242, 243, respectively. In someembodiments, the second depths W2, W3 are substantially the same, due tothe low-concentration SiGe layers 242, 243 having substantially the sameGe concentration. In some embodiments, the second depths W2, W3 aresmaller than the first depth W1 due to the low-concentration SiGe layers242, 243 having lower Ge concentration than the high-concentration SiGelayers 241. In some embodiments, the first depth W1 is larger than thesecond depths W2, W3 by a multiple of at least 1.5. In some embodiments,the outer sidewalls of the high-concentration SiGe layers 241 areplanar, convex, concave, or another shape due to characteristics of therecessing operation that forms the recesses 64.

Next, an inner spacer layer is formed to fill the recesses 64 in thenanostructures 24 formed by the previous selective etching process. Theinner spacer layer may be a suitable dielectric material, such assilicon carbon nitride (SiCN), silicon oxycarbonitride (SiOCN), or thelike, formed by a suitable deposition method such as PVD, CVD, ALD, orthe like. An etching process, such as an anisotropic etching process, isperformed to remove portions of the inner spacer layers disposed outsidethe recesses in the nanostructures 24. The remaining portions of theinner spacer layers (e.g., portions disposed inside the recesses 64 inthe nanostructures 24) form the inner spacers 74. The resultingstructure is shown in FIGS. 7A-7C.

Referring to FIG. 7D, an enlarged view showing detailed structure of theinner spacers 74 and top layer spacers 75 is shown. The inner spacers 74have generally the same lateral dimensions as the recesses 64 shown inFIG. 6D. While the inner spacers 74 are typically grown in a singleprocess, and have no discernable interface internally, geometry of theinner spacers 74 may be conveniently described in two portions: an innerportion and an outer portion. The inner portion of each inner spacer 74may physically contact outer sidewalls of the high-concentration SiGelayers 241, bottom surfaces of the low-concentration SiGe layers 242,and top surfaces of the low-concentration SiGe layers 243. Width of theinner portion of the inner spacers 74 may be equal to W1-W2, W1-W3, orW1 minus the average of W2 and W3, for example. The outer portion ofeach inner spacer 74 may physically contact the bottom surface of thenanosheets 225, the outer sidewalls of the low-concentration SiGe layers242, 243, and the top surface of either the nanosheets 225 or the fin32. The outer portion of each inner spacer 74 may further physicallycontact an inner sidewall of the epitaxial source/drain regions 82 (seeFIG. 1 , for example).

In embodiments including the low-concentration SiGe layers 242D abovethe topmost nanosheet, e.g., the nanosheet 225A shown in FIG. 7D, toplayer spacers 75 may be formed due to recessing of the low-concentrationSiGe layer 242A that occurs while forming the openings 64 of FIGS.6A-6D. Top surfaces of the top layer spacers 75 may be in physicalcontact with bottom surfaces of the gate spacers 41 (not shown in FIG.7D for simplicity). Inner sidewalls of the top layer spacers 75 may bein physical contact with the outer sidewalls of the low-concentrationSiGe layers 242A. Outer sidewalls of the top layer spacers 75 may be inphysical contact with the inner sidewalls of the epitaxial source/drainregions 82. Bottom surfaces of the top layer spacers 75 may be inphysical contact with the top surfaces of the nanosheets 225A.

FIGS. 8A-8C illustrate formation of the source/drain regions 82corresponding to act 1400 of FIG. 11 . In the illustrated embodiment,the source/drain regions 82 are epitaxially grown from epitaxialmaterial(s). In some embodiments, the source/drain regions 82 exertstress in the respective channels 22A-22C, thereby improvingperformance. The source/drain regions 82 are formed such that each dummygate structure 40 is disposed between respective neighboring pairs ofthe source/drain regions 82. In some embodiments, the spacer layer 41separates the source/drain regions 82 from the dummy gate layer 45 by anappropriate lateral distance to prevent electrical bridging tosubsequently formed gates of the resulting device.

The source/drain regions 82 may include any acceptable material, such asappropriate for n-type or p-type devices. For n-type devices, thesource/drain regions 82 include materials exerting a tensile strain inthe channel regions, such as silicon, SiC, SiCP, SiP, or the like, insome embodiments. When p-type devices are formed, the source/drainregions 82 include materials exerting a compressive strain in thechannel regions, such as SiGe, SiGeB, Ge, GeSn, or the like, inaccordance with certain embodiments. The source/drain regions 82 mayhave surfaces raised from respective surfaces of the fins and may havefacets. Neighboring source/drain regions 82 may merge in someembodiments to form a singular source/drain region 82 adjacent twoneighboring fins 32.

The source/drain regions 82 may be implanted with dopants followed by ananneal. The source/drain regions may have an impurity concentration ofbetween about 10¹⁹ cm⁻³ and about 10²¹ cm⁻³. N-type and/or p-typeimpurities for source/drain regions 82 may be any of the impuritiespreviously discussed. In some embodiments, the source/drain regions 82are in situ doped during growth. A contact etch stop layer (CESL) andinterlayer dielectric (ILD), not illustrated for simplicity, may then beformed covering the dummy gate structures 40 and the source/drainregions 82.

FIG. 9A, FIG. 9B, and FIG. 9C illustrate release of fin channels 22A-22Cby removal of the high-concentration SiGe layers 241, the mask layer 47,and the dummy gate layer 45, which corresponds to act 1500 of FIG. 11 .A planarization process, such as a CMP, is performed to level the topsurfaces of the dummy gate layer 45 and gate spacer layer 41. Theplanarization process may also remove the mask layer 47 (see FIG. 8A) onthe dummy gate layer 45, and portions of the gate spacer layer 41 alongsidewalls of the mask layer 47. Accordingly, the top surfaces of thedummy gate layer 45 are exposed.

Next, the dummy gate layer 45 is removed in an etching process, so thatrecesses 92 are formed. In some embodiments, the dummy gate layer 45 isremoved by an anisotropic dry etch process. For example, the etchingprocess may include a dry etch process using reaction gas(es) thatselectively etch the dummy gate layer 45 without etching the spacerlayer 41. The dummy gate dielectric, when present, may be used as anetch stop layer when the dummy gate layer 45 is etched. The dummy gatedielectric may then be removed after the removal of the dummy gate layer45.

The high-concentration SiGe layers 241 are removed to release thenanostructures 22. In some embodiments, the high-concentration SiGelayers 241 are removed by a selective etching process using an etchantthat is selective to the material of high-concentration SiGe layers 241,such that the high-concentration SiGe layers 241 are removed withoutsubstantially attacking the nanostructures 22. In some embodiments, theetching process is an isotropic etching process using an etching gas,and optionally, a carrier gas, where the etching gas comprises F2 andHF, and the carrier gas may be an inert gas such as Ar, He, N2,combinations thereof, or the like. After the high-concentration SiGelayers 241 are removed, the nanostructures 22 extend horizontally (e.g.,parallel to a major upper surface of the substrate 110). Variousconfigurations of the nanostructures 22 are illustrated in enlargedviews shown in FIGS. 9D-9F.

Referring to FIGS. 9D-9F, in some embodiments, the low-concentrationSiGe layers 242, 243 are removed, corresponding to act 1600 of FIG. 11 .As shown in FIG. 9D, the removal may be performed in both the firstregion 100N and the second region 100P. Removal of the low-concentrationSiGe layers 242, 243 is accomplished, in some embodiments, by a trimmingprocess that oxidizes the SiGe material of the low-concentration SiGelayers 242, 243, followed by an oxide removal process to remove theoxidized SiGe material. In some embodiments, the trimming processfurther oxidizes exposed portions of the intermixing regions 221, 222,and the oxidized portions of the intermixing layers 221, 222 are alsoremoved, such that partial intermixing regions 223, 224 remain. Thepartial intermixing regions 223, 224 are generally laterally alignedwith the outer portions of the inner spacers 74, as shown in FIG. 9D.Removal of the low-concentration SiGe layers 242, 243 and optionally theexposed portions of the intermixing regions 221, 222 reduces oreliminates positive threshold voltage shift in the GAA devices 20N, 20P.This may be desirable for PFETs with higher threshold voltage(magnitude), e.g., PFET IO transistors, and for NFETs with lowerthreshold voltage (magnitude), e.g., NFET core logic transistors.

Referring to FIG. 9E, in some embodiments, the low-concentration SiGelayers 242, 243 are removed only in the first region 100N, and arepresent in the second region 100P. Devices in the second region 100P maybe covered by a mask, such as a photoresist mask, while devices in thefirst region 100N are exposed by the mask, during the oxidation andremoval processes described above with respect to FIG. 9D. Removal ofthe low-concentration SiGe layers 242, 243 and optionally the exposedportions of the intermixing regions 221, 222 as described with respectto FIG. 9D reduces or eliminates positive threshold voltage shift in theGAA devices 20N. The presence of the low-concentration SiGe layers 242,243 in the second region 100P positively shifts the threshold voltage inthe GAA devices 20P. This may be desirable for PFETs with lowerthreshold voltage (magnitude), e.g., PFET core logic transistors, andfor NFETs with lower threshold voltage (magnitude), e.g., NFET corelogic transistors.

Referring to FIG. 9F, in some embodiments, the low-concentration SiGelayers 242, 243 are present (not removed) in both the first region 100Nand the second region 100P. As such, the positive shift in the thresholdvoltage is present in the GAA devices 20N, 20P. This may be desirablefor PFETs with lower threshold voltage (magnitude), e.g., PFET corelogic transistors, and for NFETs with higher threshold voltage(magnitude), e.g., NFET IO transistors.

An IC device may include NFETs with low threshold voltage, NFETs withhigh threshold voltage, PFETs with low threshold voltage, and PFETs withhigh threshold voltage. As such, the techniques described with respectto FIGS. 9D-9F can be combined, in some embodiments. First transistorsfor which the positive threshold voltage shift is desirable, such asPFET core logic transistors and NFET IO transistors, may be maskedduring the trimming process, so that the low-concentration SiGe layers242, 243 are present in the first transistors. Second transistors forwhich the positive threshold voltage shift is not desirable, such asPFET IO transistors and NFET core logic transistors, may be exposedduring the trimming process to remove the low-concentration SiGe layers242, 243. In some embodiments, a partial trim that merely thins, butdoes not remove entirely, the low-concentration SiGe layers 242, 243 maybe employed to achieve a partial positive threshold voltage shift, suchthat three different threshold voltages may be achieved for each of theNFETs and the PFETs.

In some embodiments, when the nanosheets 225 of the GAA device 20Nand/or the GAA device 20P are exposed following the trimming process,the nanosheets 225 are further reshaped (e.g. thinned) by anotheretching process to improve gate fill window. The reshaping may beperformed by an isotropic etching process selective to the nanosheets225. After reshaping, the nanosheets 225 may exhibit the dog bone shapein which middle portions of the nanosheets 225 are thinner thanperipheral portions of the nanosheets 225.

Next, in FIGS. 10A-10F, replacement gates 200, such as the gatestructures 200A, 200B, are formed, corresponding to act 1700 of FIG. 11. Each replacement gate 200 generally includes the first IL layer 210,the gate dielectric layer 220, the second IL layer 240, the workfunction metal layer 900, and the gate fill layer 290, as described withrespect to FIG. 1 .

Additional processing may be performed to finish fabrication of the GAAdevice 20N and/or the GAA device 20P. For example, gate contacts (notillustrated for simplicity) and the source/drain contacts 120 may beformed to electrically couple to the gate structures 200A-200B and thesource/drain regions 82, respectively, corresponding to act 1800 of FIG.11 . An interconnect structure may then be formed over the source/draincontacts 120 and the gate contacts. The interconnect structure mayinclude a plurality of dielectric layers surrounding metallic features,including conductive traces and conductive vias, which form electricalconnection between devices on the substrate 110, such as the GAA devices20N, 20P, as well as to IC devices external to the IC device 10.

Embodiments may provide advantages. The high-concentration SiGe layers241 ensure breakthrough during sheet formation. The low-concentrationSiGe layers 242, 243 reduce or eliminate intermixing between the siliconof the Si nanosheets 225 and the germanium of the SiGe nanosheetstructures 24 during thermal processes, such as annealing. Selectivetrimming, such as by a SiGe oxidation process followed by oxide removal,may be performed to remove or keep the low-concentration SiGe layers242, 243 depending on whether a positive shift in threshold voltage isdesirable (keep) or not desirable (remove). This provides an additionallever for threshold voltage tuning in GAA devices, such as the GAAdevices 20N, 20P.

In accordance with various embodiments, a device comprises a substrate;a first semiconductor channel over the substrate, comprising: a firstnanosheet of a first semiconductor material; a second nanosheet of asecond semiconductor material in physical contact with a topside surfaceof the first nanosheet; and a third nanosheet of the secondsemiconductor material in physical contact with an underside surface ofthe first nanosheet; and a first gate structure over and laterallysurrounding the first semiconductor channel, and in physical contactwith the second nanosheet and the third nanosheet.

In accordance with various embodiments, a method comprises forming afirst multi-layer semiconductor stack on a substrate, comprising a firstsilicon germanium layer, a second silicon germanium layer, and a thirdsilicon germanium layer; forming a silicon layer on the firstmulti-layer semiconductor stack; forming a second multi-layersemiconductor stack on the silicon layer, comprising a fourth silicongermanium layer, a fifth silicon germanium layer, and a sixth silicongermanium layer; forming a first semiconductor fin and a secondsemiconductor fin by patterning the first multi-layer semiconductorstack, the silicon layer, and the second multi-layer semiconductorstack; removing the second silicon germanium layer and the fifth silicongermanium layer of the first and second semiconductor fins by a firstetching process that is more selective to the second and fifth silicongermanium layers than to the first, third, fourth and sixth silicongermanium layers; and forming a first gate structure in a first openingformed by the first etching process.

In accordance with various embodiments, a device comprises a firsttransistor on a substrate; and a second transistor on the substrate,comprising: a first silicon nanosheet; a second silicon nanosheet; afirst silicon germanium layer physically contacting a major surface ofthe first silicon nanosheet facing the second silicon nanosheet; and asecond silicon germanium layer physically contacting a major surface ofthe second silicon nanosheet facing the first silicon nanosheet.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A device, comprising: a substrate; a firstsemiconductor channel over the substrate, including: a first nanosheetof a first semiconductor material; a second nanosheet of a secondsemiconductor material in physical contact with a topside surface of thefirst nanosheet; and a third nanosheet of the second semiconductormaterial in physical contact with an underside surface of the firstnanosheet; and a first gate structure wrapping around the firstsemiconductor channel and in physical contact with the second nanosheetand the third nanosheet.
 2. The device of claim 1, wherein the firstsemiconductor material is silicon, and the second semiconductor materialis silicon germanium having germanium concentration in a range of about10% to about 25%.
 3. The device of claim 1, wherein thickness of thefirst nanosheet is greater than at least one of thickness of the secondnanosheet or thickness of the third nanosheet by a multiple in a rangeof about 2 to about
 16. 4. The device of claim 1, further comprising: asecond semiconductor channel over the substrate, the secondsemiconductor channel including a fourth nanosheet of the firstsemiconductor material; and a second gate structure wrapping around thesecond semiconductor channel and in physical contact with the fourthnanosheet.
 5. The device of claim 4, wherein vertical spacing betweenthe first nanosheet and the fourth nanosheet is greater than at leastone of thickness of the second nanosheet or thickness of the thirdnanosheet by a multiple in a range of about 3.5 to about
 26. 6. Thedevice of claim 4, wherein: the first semiconductor channel is includedin a P-type field effect transistor; and the second semiconductorchannel is included in an N-type field effect transistor.
 7. The deviceof claim 4, wherein: the second nanosheet and the third nanosheet are inphysical contact with a first interfacial layer of the first gatestructure; and the fourth nanosheet is in physical contact with a secondinterfacial layer of the second gate structure.
 8. A device, comprising:a first transistor of a first type, the first transistor including: afirst vertical stack of first nanostructure channels, each including afirst layer of a first semiconductor material; and a first gatestructure in contact with the respective first layers of the firstnanostructure channels; and a second transistor of a second typedifferent from the first type, the second transistor including: a secondvertical stack of second nanostructure channels, each including: asecond layer of the first semiconductor material; and a third layer of asecond semiconductor material different from the first semiconductormaterial; and a second gate structure in contact with the respectivethird layers of the second nanostructure channels, the second gatestructure being vertically separated from the respective second layersby the respective third layers.
 9. The device of claim 8, wherein thefirst semiconductor material is silicon, and the second semiconductormaterial is silicon germanium having germanium concentration in a rangeof about 10% to about 25%.
 10. The device of claim 8, wherein each ofthe first nanostructure channels further includes a fourth layer of thesecond semiconductor material, the first gate structure being in contactwith the respective fourth layers of the first nanostructure channels.11. The device of claim 8, wherein the third layer extends along anentire length of the second gate structure in a first direction.
 12. Thedevice of claim 11, wherein the third layer extends past the second gatestructure in the first direction.
 13. The device of claim 8, furthercomprising: an inner spacer having a sidewall in direct contact with thefirst gate structure in a lateral direction, and having an upper surfaceseparated from one of the first layers by a fourth layer of the secondsemiconductor material.
 14. A device, comprising: a first transistor ona substrate; and a second transistor on the substrate, the secondtransistor including: a first silicon nanosheet; a second siliconnanosheet; a first silicon germanium layer physically contacting a majorsurface of the first silicon nanosheet facing the second siliconnanosheet; and a second silicon germanium layer physically contacting amajor surface of the second silicon nanosheet facing the first siliconnanosheet.
 15. The device of claim 14, wherein the second transistorfurther comprises: a first inner spacer having a first outer portion inphysical contact with the first silicon nanosheet and the second siliconnanosheet, and a first inner portion between the first silicon germaniumlayer and the second germanium layer; wherein the first outer portion isthicker than the first inner portion.
 16. The device of claim 15,wherein the first transistor further comprises: a third siliconnanosheet configured as a third channel; a fourth silicon nanosheetconfigured as a fourth channel; a second inner spacer having an outerportion in physical contact with the third silicon nanosheet and thefourth silicon nanosheet, and an inner portion thinner than the outerportion of the second inner spacer; and a second interfacial layer inphysical contact with the inner portion of the second inner spacer andthe third silicon nanosheet.
 17. The device of claim 15, wherein thesecond transistor further comprises: a first interfacial layer inphysical contact with the inner portion of the first inner spacer andthe first silicon germanium layer.
 18. The device of claim 14, whereinthe first silicon nanosheet further comprises: a first intermixingregion in physical contact with the first silicon germanium layer,wherein germanium concentration in the first intermixing region is lessthan about 5%.
 19. The device of claim 14, wherein the second transistorfurther comprises: a third silicon germanium layer physically contactinganother major surface of the first silicon nanosheet facing away fromthe second silicon nanosheet, wherein the first silicon nanosheet is themost distal silicon nanosheet from the substrate of a gate-all-aroundfield effect transistor; a gate spacer above the first siliconnanosheet; and a top layer spacer in physical contact with the gatespacer and the first silicon nanosheet.
 20. The device of claim 14,wherein the second transistor further comprises: a semiconductor finunder the first and second silicon nanosheets; and a fourth silicongermanium layer physically contacting a surface of the semiconductor finfacing the first and second nanosheets.